I was very happy when I discovered, that a beta of the new version of OpenELEC was released. You can get it from the official OpenELEC website.
The new version (even though it is still beta) works pretty fine on my Xtreamer Ultra. PXE-boot from NFS works out of the box, Splash-Screen is working properly, Boot-Time is very fast (seems like Realtek-Firmware is now on the right location by default).
A big thanks to all of the guys who develop this great software.